The use of intramedullary pins is a known and established form of treatment for fractures in long bones. Intramedullary pins include fixing holes or bores at the proximal and distal ends of the intramedullary pin (the implant), and screws can be inserted into the fixing holes or bores in order to fix or attach the intramedullary pin to the fractured bone.
One problem with conventional intramedullary pins is that the implanted intramedullary pin is deformed by the shape of the long bone as the intramedullary pin is introduced into the bone. Due to this deformation, the position and orientation of the distal end of the intramedullary pin is in principle unknown and, therefore, the position and orientation of the distal fixing bore also is unknown.
In order to insert the fixing screws into the distal fixing bore, the position of the distal fixing bore must be determined, and this is conventionally achieved with the aid of x-ray images. Surgeons generally use a C-arc fluoroscopy apparatus to determine the position of the pin and/or bore, thus facilitating the insertion of the screw into the distal fixing bore.
A disadvantage of this conventional approach is that it exposes the patient and operating team to an undesirable radiation load. A further disadvantage in using C-arcs is the lack of a third dimension. On the basis of intra-operatively produced x-ray projections, even from a number of angles, a sufficient spatial orientation for reliably controlling the introduction of screws in all three dimensions and all degrees of freedom generally is lacking.
In order to solve these problems, an attempt has been made to use an optical target system such as that described in U.S. Pat. No. 5,417,688, wherein a light source is attached to the distal end of the intramedullary pin, and the light source is used to determine the orientation of the distal fixing bore. The intent is to use the light radiation transmitted onto the surface of the patient's body to orient a drill. The light source, however, can be difficult to identify, resulting in inaccurate identification of the fixing bore location.
Alternatively, a suggestion also has been made to use magnetic targeting systems such as, for example, those described in U.S. Pat. Nos. 6,162,228, 5,411,503 and 5,584,838, in which a special configuration of magnets is arranged or a predetermined magnetic field is generated at the distal end of the intramedullary pin. The magnetic field then is used to determine the orientation of the distal fixing bore. The surgeon uses a navigation tool provided with a magnetic sensor system in order to fix the intramedullary pin. A drawback to these systems is that they can be costly and relatively susceptible to faults. The latter particularly applies if steel pins are used.
Another common approach is based on using existing navigation methods that provide for tracking a pin, and integrating C-arc into navigation. In these systems, experience has shown that solely tracking the pin, without taking into account the deformation, generally leads to incorrect bores, despite the use of fluoroscopy in combination with navigation.
Another disadvantage of these known methods is that they are based on the use of radiation (C-arc) for imaging, and the position of the through-bores in the image is determined manually and imprecisely (due to misinterpretation of the displayed projection, for example). In order to be able to navigate in the C-arc images, the images must be provided in a calibrated form (rectified and with the spatial position of the projection known when recording) and the bone structure in which the intramedullary pin is situated must be provided with a reference array (additional invasiveness).